The present invention relates to telecommunications, and more particularly to apparatuses and techniques for transmitting and receiving radio frequency signals while reducing VCO-pulling sensitivity.
In radio frequency communications, information to be communicated is typically generated at an initial relatively low, so-called “baseband” frequency. This baseband frequency signal is then processed in a way that results in the information being imposed on a much higher, radio frequency signal. This processing is often referred to as “up-converting.” In the receiver, a reverse process (called “down-converting”) is performed on the received radio frequency signal to re-create the original baseband frequency signal.
Many different transmitter and receiver architectures are known. In some, the initial signal (baseband for transmitters, radio frequency for receivers) is first up- or down-converted to one or more intermediate frequencies. These intermediate frequency (IF) signals may then be subjected to further processing before ultimately being up- or down-converted to the respective radio frequency (RF) or baseband signal (depending on whether transmission or reception is being performed).
In these so-called “non-zero IF” up- or down-conversion systems, undesired signals oscillating at an image frequency can leak into the system, creating spurious IF signals. A measure of the quality of a transmitter or receiver is its image rejection ratio, which is defined as the ratio of (a) the IF signal level produced by the desired input frequency to (b) that produced by the image frequency. In most applications found in modem communications, however, it is very difficult to design a non-zero IF architecture that meets an imposed image rejection requirement. For this reason, designers often choose a zero-IF up- and down-conversion approach, in which baseband and radio frequency signals are converted directly from one to the other.
Another aspect of modem telecommunications is how the information will be imposed on the radio frequency signal. A common approach is to impose some of the information on a first, in-phase signal, and to impose the remainder of the information on a second, quadrature signal. The in-phase and quadrature signals are then combined to form the final signal that will be communicated between the transmitter and the receiver. The resultant analog signal is one in which each combination of phase and (possibly) amplitude represents one of a number of n-bit patterns (n is an integer). (The term “quadrature” pertains to the phase relationship between two periodic quantities varying with the same frequency when the phase difference between them is one-quarter of their period; that is, the two periodic quantities are π/2 radians out of phase with respect to one another.) Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) are two well-known examples of this type of modulation.
FIG. 1 is a block diagram of a conventional zero-IF transceiver 100. The transceiver 100 includes a transmitter 101 and a receiver 103. As can be seen in the figure, for the sake of efficiency the transceiver 100 includes a synthesizer 105 that is shared by the transmitter 101 and the receiver 103. It should be recognized that, in general, transmitters and receivers can be constructed separately, each with its own synthesizer.
Focusing now on the transmitter 101, information in the form of bits to be transmitted are supplied to digital logic 107. The digital logic 107 may perform a number of functions that are not illustrated here, such as generating redundant bits in accordance with a Forward Error Correction (FEC) scheme. One function that the digital logic 107 does perform in this example is generating quadrature modulation signals A(t)·cos(θ(t)) and A(t)·cos(θ(t)+π/2) from the supplied input bits. A(t) and θ(t) will depend on the type of modulation used in the transmitter (e.g., PSK, FSK, ASK, etc.). One of the signals A(t)·cos(θ(t)) and A(t)·cos(θ(t)+π/2) is supplied to an in-phase transmit path, and the other of the two generated signals is supplied to a quadrature-phase transmit path. It will be observed, then, that the digital logic 107 ensures a π/2 radians phase difference between the bits supplied to the in-phase and quadrature-phase transmit paths. In each of these paths, the bits supplied by the digital logic 107 are converted to analog form by a digital to analog (D/A) converter 109. The analog signal supplied by the digital to analog converter 109 is then conditioned for transmission by a low pass filter (LPF) 111. The conditioned analog signal is then directly up-converted to the radio frequency that will be used for transmission by mixing the signal (via a mixer 113) with a radio frequency signal generated by the synthesizer 105.
The quadrature relationship between the two transmit signal paths is also accomplished by supplying a first radio frequency signal to the mixer 113 associated with the in-phase transmit path, and a second radio frequency signal to the mixer 113′ associated with a quadrature-phase transmit path, wherein there is a π/2 radians phase difference between the first and second radio frequency signals. The in-phase and quadrature-phase signals supplied by the mixers 113 and 113′ are then combined 115 and supplied to a power amplifier 117, which boosts the strength of the signal so that it can be transmitted through an antenna 119.
In the conventional transceiver 100, the synthesizer 105 generates the radio frequency signals to be supplied to the mixers 113, 113′ by means of a phase locked loop. Accordingly, a reference signal is supplied to phase difference circuitry 121 whose output represents the phase difference between the reference signal and a signal related to the output of the synthesizer 105. The phase difference signal supplied by the phase difference circuitry 121 is then conditioned by a low pass filter 123. The conditioned signal is used to control the frequency of an output signal generated by a voltage controlled oscillator (VCO) 125. In this case, the output signal supplied by the VCO 125 oscillates at twice the desired radio frequency so it is supplied to a divide-by-two circuit 127 which generates both the in-phase and quadrature-phase radio frequency signals needed by the transmitter 101 and (in this example) the receiver 103. These signals represent the outputs of the synthesizer 105. One of the output signals supplied by the divide-by-two circuit 127 is also supplied to a divide-by-N frequency divider 129, whose output is 1/N times the frequency of the synthesizer output signals. The frequency-divided signal supplied at the output of the divide-by-N frequency divider 129 is the above-mentioned signal related to the output of the synthesizer 105, which signal is supplied as one of the inputs to the phase difference circuitry 121.
The relevant features of the receiver 103 essentially perform the inverse operations of those found in the transmitter 101, and are therefore not described here.
One problem encountered in zero-IF transmitter arrangements is the occurrence of signal coupling between the radio frequency power amplifier and the tank circuit of the VCO by capacitive or inductive means. This problem, which is called VCO-pulling, is very difficult to avoid when the VCO is designed to run at the same frequency as the frequency of the output signal to be transmitted. This is an especially large problem went operating frequencies are in the gigahertz range.
One way of decreasing this pulling is to choose a VCO frequency unequal to the frequency of the output-signal. A common choice is fVCO=2·fRF, as illustrated in FIG. 1. Running the VCO at twice the frequency of the transmitted radio frequency signal also makes it easy to generate the quadrature signals which are required for up-conversion of the modulated signal, which is generated at 0 Hz (see the divide by two circuit 127 illustrated in FIG. 1). However, even for this configuration, harmonics of the output signal (e.g., those present in the supply line or those generated by distortion of the RF-signal in the power amplifier itself) still cause pulling of the VCO. This is illustrated by the signal leakage path 131 illustrated in FIG. 1, which permits spurious signals having frequencies at
      n    ·          (                        ω          0                +                              ⅆ                          θ              ⁡                              (                t                )                                                          ⅆ            t                              )        ,where n is an integer, to leak from the power amplifier 117 to the VCO 125.
It is therefore desirable to provide apparatuses and methods that avoid any pulling of the VCO by radio frequency signals present in the transmitter, because this VCO-pulling deteriorates the spectral purity of the transmitter output signal and consequently prevents the transmitter from satisfying test specification requirements covering transmitter modulation accuracy. It is further desirable to avoid this VCO pulling because the resultant local oscillator signals have an undesirable spectrum which, if supplied to the mixer of a receiver, can reduce the receiver's ability to handle interfering signals.